Front Brakes Locking On After Car Idles.

When it locks up have you opened the bleeders to see if the pressure lets off? If so is it one wheel or both? Did you compare the rebuilt master cylinder to the old one and did you check the depth of the hole in the piston where the pushrod from the booster goes to see if it was the same? There is a spec and adjustment on the pushrod from the booster to the master cylinder. If the compensating ports in the master cylinder are not being allowed to return far enough the fluid from the lines will not return to the reservoir. What it sounds like to me is you have one of a few things going on. Yes the fluid expands under heat, but you either have a bad master cylinder (whether the bore in in the piston isn't deep enough, or some internal issue) or the adjustment of the pushrod from the booster is not correct and preventing the master cylinder piston from fully returning to expose the compensating ports. I don't have my manual handy but I but will have it here in a few and I will look up that information on the rod.